A Mailbox for Fish

Here's your nightly math! Just 5 quick minutes of number fun for kids and parents at home. Read a cool fun fact, followed by math riddles at different levels so everyone can jump in. Your kids will love you for it.

A Mailbox for Fish

It’s pretty amazing that you can write a letter to a friend, write an address hundreds of miles away on the envelope, and within days your friend gets your letter in the mailbox. How much more exciting when that mailbox is underwater! The fishing town of Susami, Japan took mail to the next level by setting up an underwater mailbox. It’s a real mailbox, except it’s more than 30 feet deep in the ocean. The town post office put it in place in 1999 to stir up excitement among visitors, and so far about 32,000 pieces of mail have been sent through it. To mail a letter, you have to buy a waterproof postcard, write on it with an oil-based marker, and then swim down to stick it in the mailbox. So it’s been used mostly by divers. The post office checks the box once every few days, so if you happen to visit Susami, you can be sure any mail you send will reach your friends – as long as a fish doesn’t grab it first.

Wee ones: The post office uses 2 mailboxes that take turns going underwater, so each can be cleaned of rust and repainted. If they use mailbox A, then mailbox B, then A, then B, which one do they use the next 3 times?

Little kids: If the box is 30 feet underwater and you’re able to dive 50 feet deep, can you reach the box? Bonus: If the post office there checks the box every 5 days, starting on Sunday, then Friday, what are the days of week for the next 2 times they check it?

Big kids: If you lived in Susami and had to walk 100 feet to the dock and dive 30 feet down to mail a postcard, how many feet would you travel in total round trip? Bonus: If the box was installed nearly 16 years ago and they’ve gotten 32,000 pieces of mail, about how many letters per year is that?

Answers:Wee ones: A, then B, then A.

Little kids: Yes, since 50 is more than 30. Bonus: Wednesday, then Monday…it’s the same as counting back 2 days.

Big kids: 260 feet. Bonus: About 2,000 pieces per year.

About the Author

Laura Bilodeau Overdeck is founder and president of Bedtime Math Foundation. Her goal is to make math as playful for kids as it was for her when she was a child. Her mom had Laura baking while still in diapers, and her dad had her using power tools at a very unsafe age, measuring lengths, widths and angles in the process. Armed with this early love of numbers, Laura went on to get a BA in astrophysics from Princeton University, and an MBA from the Wharton School of Business; she continues to star-gaze today. Laura’s other interests include her three lively children, chocolate, extreme vehicles, and Lego Mindstorms.